The present invention relates to tapping units intended to be mounted on a machine tool. The invention relates in particular to tapping units that can be used on a machine tool performing one or more machining operations on parts continuously.
For example, this kind of tapping unit can be used on a part-forming press to produce one or more threads on a part.
Taps for cutting threads on a press are already known in the art. For example, the document EP 0 999 006 describes a tap of the above kind including a casing intended to be fixed opposite a part to be tapped on a machine tool such as a press. The casing includes an interior housing which accommodates a pattern in the form of a bush with a tapped bore prevented from rotating. The pattern receives in its bore a tap-carrier which itself includes means for receiving and retaining a tap. The tap-carrier has a threaded section functionally engaged in the tapped bore of the pattern. The tap-carrier has a proximal driving section with a polygonal cross section which slides in a bore with a polygonal cross section in a drive shaft rotatably mounted in the casing. The drive shaft is rotated by a mechanical transmission driven by a motor.
The mechanical transmission includes a flexible shaft that connects the mobile members of the casing to a remotely sited motor.
The casing is mounted on the part-carrier of the machine with elastic means between them such as blocks of elastic material. The blocks of elastic material must nevertheless have sufficient stiffness to position the tap correctly at the start of machining. This reduces the possibilities for radial and axial displacement of the tap and unwanted movements of the part during machining generate relatively high forces on the tap during tapping, and so compromise the quality of the tapping effected, reduce the service life of the taps, and introduce the risk of the tap-carrier loosening.
What is more, the flexible drive shaft applies a non-negligible force or torque to the casing, modifying its position and thereby affecting the position of the tap at the start of machining. This necessitates a further increase in the stiffness of the blocks of elastic material joining the casing and the part-carrier of the machine, so compromising the freedom of movement of the tap.
The tapping head formed by the casing and the parts that it contains has a relatively high inertia, which reduces the capacity for fast movement of the tap relative to the part to be machined during tapping.
These drawbacks are more serious when tapping small diameters.
The document DE 197 29 263 A describes a tapping unit including at the same time means allowing slight and limited radial movement of the tap in the casing around a mean radial position, and means allowing limited retrograde axial movement of the tap toward the inside of the casing from a reference position and against the action of return spring means. To this end, the tap is mounted in a clamping bush which is mounted with some radial clearance in an operating shaft whose central portion is coupled to a conical drive wheel and whose proximal portion is threaded to engage in the tapped bore of a pattern which is mounted to slide axially in the casing and pushed forward by a spring so that it abuts against a shoulder of the casing.
In the normal tapping position, the pattern bears against the shoulder of the casing and on the one hand the tap can therefore oscillate radially with the bush, and can retract toward the interior of the casing, against the thrust of the spring. On the other hand, the tap cannot move in the direction toward the exterior of the casing.
The above device is known to facilitate engaging the tap in a preliminary hole in the part to be tapped, and to allow retrograde movement of the tap if there is no such hole or if the hole is offset too far.
However, the above device is not designed to follow unwanted movements of the parts to be machined during mass production tapping, and there is then a tendency for defective tapping, reduced tap service life, and loosening of the tap-carriers.
The problem addressed by the present invention is to improve the quality of the tapping effected by tapping units mounted on a machine tool such as a press. The invention stems from the observation that tapping defects are generated by unwanted movements of the part to be machined on the part-carrier, which movements induce non-negligible forces between the part to be machined and the tap. On mass production machine tools, the parts to be machined are pressed onto the part-carrier by appropriate retaining means, but are not retained with a constant and permanent pressure, in particular at the start of tapping. There are therefore unwanted movements in the direction of separation of the part from the casing of the tapping unit. These movements, although of very limited amplitude, are inevitable and frequent, and are related to the kinematics of the cutting tools. It appears that these unwanted separation movements significantly impact on the service life of the tap, the quality of tapping, and how effectively the tap is clamped in the tap-carrier.
The basic idea of the invention is to enable limited forward movements of the tap out of the tapping head casing, the casing being rigidly fixed to the support of the machine, so that the tap can freely follow the frequent unwanted movements of the part to be tapped. What is more, the moving parts constitute a subassembly of low mass and low inertia, capable of following fast unwanted movements of the part to be machined relative to the part-carrier.
Accordingly, the invention provides a tapping unit including:
a casing adapted to be fixed facing a part to be machined on a support of a machine tool such as a press and having an interior housing.
a pattern, including a bush with a tapped bore, inserted axially in the interior housing of the casing and prevented from rotating by immobilizing means.
a tap-carrier, including means for receiving and retaining a tap, having a threaded section functionally engaged in the tapped bore of the pattern, and having a drive section,
a drive shaft, rotatably mounted in the casing, having a bore receiving the drive section of the tap-carrier, which is adapted to slide longitudinally in it, and driven in rotation by a mechanical transmission driven by a motor.
means for allowing slight and limited radial movement of the tap in the casing about a mean radial position,
means for allowing limited axial forward movement of the tap toward the exterior of the casing from a reference position and against the action of advance return spring means.
In a practical embodiment, the means for allowing limited axial forward movement of the tap include:
an axial bore in the casing in which the pattern can slide longitudinally between the reference position and a retracted proximal position.
a posterior distal shoulder in said axial bore in the casing.
an anterior distal shoulder on the pattern, facing the posterior distal shoulder in the bore, with an appropriate axial distance between said shoulders when the pattern is in a reference position.
an advance return spring means comprising an advance return spring inserted in the axial bore in the casing between the anterior distal shoulder of the pattern and the posterior distal shoulder of the bore in order to push the pattern axially back in the retraction direction.
The unit preferably further includes return radial spring means for returning the tap-carrier to a centered radial position in the casing.
In a simplified embodiment, the means allowing slight and limited radial displacement of the tap in the casing include:
the fact that the drive section is a proximal section of the tap-carrier,
the fact that the tap is held fixedly in the tap-carrier,
a slight radial clearance between the polygonal cross section bore in the drive shaft and the proximal drive section of the tap-carrier,
an appropriate radial clearance between the pattern and the axial bore in the casing that guides it laterally, so that the tap-carrier, the tap and the pattern form an autonomous subassembly able to move in the fixed casing with a slight and conical oscillatory movement.
To increase further the capacity to absorb unwanted movements of the part to be machined, the tapping unit according to the invention preferably further includes means for allowing limited axial retrograde movement of the tap toward the interior, of the casing from the reference position, and a retraction return spring which pushes the pattern axially in the forward direction.
In a practical embodiment, the means for allowing limited axial retrograde movement of the tap include:
a proximal posterior shoulder on said axial bore in the casing,
a corresponding proximal posterior shoulder on the pattern,
said retraction return spring, inserted in the interior housing of the casing between the proximal posterior shoulder of the pattern and a front bearing surface of the casing, in order to push the pattern axially in the forward direction, the thrust of the retraction return spring being inhibited by the proximal posterior shoulder of the axial bore in the casing when the pattern is at or downstream of its reference position.
Sealing means are preferably disposed between the pattern and the tap-carrier at the distal outlet of the bore in the pattern, in order to oppose polluting fluids reaching the co-operating threads of the tapped bore in the pattern and the threaded section of the tap-carrier.
In another aspect, the invention provides a press for forming and machining parts, characterized in that it includes a tapping unit as defined hereinabove, the casing of the tapping unit being fixed to the part-carrier of the press with the tap facing toward the part to be machined, so that the freedom of radial and axial movement of the tap allows it to follow unwanted movements of the part to be machined on the part-carrier during tapping.